Add a changes file for removing old consensus methods

(Ticket 24378; proposal 290.)

changes/24378

+  o Removed features:
+
+    - Directory authorities will no longer support voting according to any
+      consensus method before consensus method 25. This keeps authorities
+      compatible with all authorities running 0.2.9.8 and later, and does
+      not break any clients or relays. Implements ticket 24378 and
+      proposal 290.
+